Turboprop gas turbine engines for aircraft which use propellers to provide propulsion have blade angle control systems to control the pitch of the propeller blades. The blade angle control system is actuated by hydraulic fluid which is supplied under pressure by a pump.
The pump used for such systems is sized to meet the largest load requirements of the blade angle control system. The hydraulic fluid flow requirements for these larger loads are significantly higher than the flow requirements of the blade angle control system during steady-state operation. The pump therefore often supplies more fluid to the blade angle control system than is required during steady-state operation. A significant amount of this excess fluid is diverted away from the blade angle control system toward the pump's inlet by a suitable valve. This can result in an undesirable increase in the temperature of the oil supplied to the pump.